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[Jim Williams]

Bruno Putzeys wrote on Thu, 30 November 2006 08:56

Jim Williams wrote on Thu, 30 November 2006 17:36 Just might be. Pop in a LM6172 into a circuit using the tired 1975 vintage NE5532, albit with compensation and psu treatment and you will find that 3000 v/us slew rate part really does make a difference. It definately sounds much more open than either the LM4562 or the AD8599, although the noise is higher. Similar question: given that slew rate is not the only performance spec in which the 6172 is better than the 4562, why attribute the sonic advantage to slew rate? Higher slew rates are found in op amps with greater GBW. As an example, I'll refer to PSRR once again. PSRR in a miller opamp equals loop gain. At frequencies above the dominant (real) pole, loop gain scales linearly with GBW and so does PSRR. You can (easily) convince me that op amps with higher GBW will sound better, but that alone is not enough to draw the conclusion that specifically slew rate is involved.

Slew rate is one factor. It's also a reflection of the speed and bandwidth of the process transistors. I had an interesting discussion with an engineer at National Semi about the 4562 and other fast opamps from them like the 6172, 6171, 7171, etc. They claim crossover distortion is reduced in high speed opamps because the transistor's switching frequncy is so fast the zero cross point is of a shorter duration.

I've found that fast opamps with decent slew rates which have very high open loop gain at 10k hz, around 85~90 db, sound better than opamps with a 3k+ slew rate and 60~70 db open loop gain at 10k hz. Sonically, one will hear low level transients and details missing from lesser opamps without the huge open loop gain in the audio band.

[Jim Williams]

Larrchild wrote on Fri, 01 December 2006 10:11

Jim Writes: Quote: BTW, I believe Buddy Holly's slew rate was 12 v/ us. Not through the input iron and output iron and magnetic recorders of the day. Hardly anyone liked that custom Cherokee console with video opamps in it in the 80's if I recall. Hmm.

Yes there was transformer slowing, it was an example. The slew rate of analog tape at 15 IPS is way worse. As I mentioned before, early high speed opamps had lot's of distortion. Still, some revere them. Consider the worshiped Ampex ATR's with their 1971 vintage LM318 70 v/ us slew rate opamps, folks love these things. I would believe that Cherokee console would sound lovely today with some of these improved opamps refitted in it.

[maxdimario]

low slew rate is perhaps a problem for the circuit's inner workings but not for the music.

[Larrchild]

Jim:

Quote:

Consider the worshiped Ampex ATR's with their 1971 vintage LM318 70 v/ us slew rate opamps, folks love these things. I would believe that Cherokee console would sound lovely today with some of these improved opamps refitted in it.

Interesting factoid regarding the ATR fast-slew opamp circuit people do love. The Cherokee example was used because I remember it as being the first time someone went "Let's build a desk with fantastic bandwidth and slew-rate" and after the fanfare, most people promptly went back to the A Range Trident. Perhaps the idea was not mature yet, and what you say is: "Give fast opamps a listen these days".

So I will!

[maxdimario]

it is MUCH easier to make a console with slow opamps than fast opamps..
fast opamps will not stand certain approximations in circuit-board and power supply designs..

[Larrchild]

Yet, for 50+ years, video engineers have amplified and mixed 5 Mhz signals thru a gaggle of routing, and managed to keep em stable and phase-coherent, so people do it all the time.

[dcollins]

Larrchild wrote on Sat, 02 December 2006 19:25

Yet, for 50+ years, video engineers have amplified and mixed 5 Mhz signals thru a gaggle of routing, and managed to keep em stable and phase-coherent, so people do it all the time.

RF actually has to work.  There are no Radiophiles, telling you the wire has some magic property previously unknown to Science.

Fwiw, when we did blind tests of IC opamps, this one seemed to win.

 http://www.datasheets.org.uk/pdf/mc34081-datasheet/mc34081-d atasheet.html

Nothing special in the specs, slow and relatively noisy by modern standards. Can't drive 600R, etc.

DC

[Bob Olhsson]

dcollins wrote on Sat, 02 December 2006 23:24

...RF actually has to work.  There are no Radiophiles, telling you the wire has some magic property previously unknown to Science...

This can cut both ways!

I attended a SMPTE meeting in San Francisco about one of the first digital HDTV facility installations. Apparently some of the vendors hadn't felt they had enough digital expertise so they went out and hired some digital audio experts to help with their interface and wiring design. Soon after the facility went into operation, experienced producers began pointing at their monitors horrified wanting to know what's THAT!

It turned out to be jitter caused by techniques that are commonly used with digital audio. The "experts" had thrown something together without really understanding what they were doing and no longer had ABX tests to hide behind when somebody complained.

[dcollins]

Bob Olhsson wrote on Sun, 03 December 2006 05:10

It turned out to be jitter caused by techniques that are commonly used with digital audio. The "experts" had thrown something together without really understanding what they were doing and no longer had ABX tests to hide behind when somebody complained.

Ok, but I bet those "experts" solved the problem using engineering, not by painting the camera lens with a green felt pen to cure the j*tter!

DC

[bruno putzeys]

When I say "tons of feedback" I mean lots of loop gain. Loop gain may be loosely defined as the difference between open loop gain and closed loop gain.

However, low closed loop gain does not necessarily imply high loop gain because you also need to know the open loop gain.

If you want to stand a reasonable chance at transparency (ie. no audible difference between input and output), shoot for a minimum of 60dB of loop gain at all audio frequencies. With normal circuit techniques open loop THD figures of 0.1% are realistic so the end result will be in the order of 0.0001%.

If the circuit you're designing has a lot of gain (mic pre), don't compensate your amps for unity gain. Consider distributing the gain among the stages and if possible run a higher order loop around the whole thing. This will make the compound gain of all stages available to fight the distortion of all stages at once. Works like a charm, and sonically it proves that minimalism is a trivial pursuit.

[maxdimario]

That is a matter of opinion.

I could 'prove' the opposite with listening tests, as i have done before... to people who know nothing about pro audio as well as people who mix.

[bruno putzeys]

You may note that I am saying transparency *requires* strong loop control, I am not saying the converse. If you have tested amps with strong loop control that are patently not transparent (because you could identify it in a double blind trial), that's not even news to  me. On the other hand, if you could show another amplifier to be perfectly transparent in a listening test, I would be hugely surprised if it measured anything but perfect.

[maxdimario]

You are correct...that's the point, an amplifier such as above will not sound exactly like the source, it will add something.

but that something it adds is negligeable when you weigh it against what it does NOT take away.

So there is no need to have such low distortion if it causes the music to lose that immediacy which you get by standing in front of an artist.

once you reduce distortion artifacts to -60 dB by using feedback it's obvious that the amp will sound 'neutral' in the sense that you will have very low thd and ruler flat response amongst other things.

unfortunately, there is a fine element which dissapears. Basically the 'liveness' of which I was writing about in the posts above.

Like I said.. it can't be proven over internet.. at least by me.

[Andy Peters]

maxdimario wrote on Tue, 05 December 2006 07:08

That is a matter of opinion. I could 'prove' the opposite with listening tests, as i have done before... to people who know nothing about pro audio as well as people who mix.

Hey, Max,

Can you PLEASE learn how to use the "QUOTE" button when replying to a message?

Every single post you've made in this thread is a reply to the first post, and since you never quote, I (and others, I'm sure), have NO IDEA which post you're actually replying to.

Thanks.

-a

[maxdimario]

sorry.